The long term objective of this proposal is to determine the predominant mechanisms of pediatric brain tumor resistance to alkylating agent based chemotherapy in order to develop strategies to combat resistance. The hypothesis to be tested is that repair of methylated bases by 3- methyladenine DNA glycosylase (MAG) and repair of abasic sites by apurinic/apyrimidinic endonuclease (Ap endo) are major factors in tumor resistance to alkylating agents. The relationship between repair and cellular resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), temozolomide and methyl methanesulfonate (MMS) will be examined in a panel of pediatric brain tumor cell lines. The contribution of MAG and Ap endo activity will be directly evaluated by examining the change in chemosensitivity when these repair activities are suppressed using antisense oligonucleotides. The contribution of MAG and Ap endo activity to resistance will be compared to the contribution of O6-methylguanine- DNA methyltransferase (MGMT) in the same cell lines. MGMT is a proven contributor to alkylating agent resistance; however, its contribution relative to other repair pathways has not been extensively studied. Assaying the chemosensitivity of cells depleted of MGMT activity with the inhibitor 6-benzylguanine (O6-BG) and simultaneously depleted of MAG and/or Ap endo activity by treatment with antisense oligonucleotides will provide information concerning the relative contribution of these repair enzymes to alkylating agent resistance and information concerning the relative lethality of the different adducts repaired by these enzymes. In addition, MAG and Ap endo activities together with MGMT activity will be examined in newly diagnosed and recurrent pediatric brain tumors of different histologic types and grades. The prognostic value of these enzyme activities will be evaluated by examining their relationship to response to therapy and time to tumor progression. The proposed cell line study will test mechanism, directly address the hypothesis that repair of alkylated bases by MAG and repair of abasic sites by Ap endo are important factors in cellular resistance. The tumor tissue studies will reveal the prognostic value and the extent of tumor heterogeneity of MAG, Ap endo and MGMT activities. The cell line and tissue data will add to our understanding of tumor resistance to alkylating-agent-based chemotherapy and aid in designing more effective chemotherapy protocols.